A polarizing plate has been usually used in a structure, in which a protective film is stacked on both surfaces of a polarizer formed of a polyvinyl alcohol (hereinafter, referred to as ‘PVA’)-based resin dyed with a dichroic dye or iodine. In this case, a triacetyl cellulose (TAC)-based film has been frequently used as the protective film because the film is excellent in optical transparency or moisture permeability.
Meanwhile, as a liquid crystal display device has recently been developed into a mobile device such as a laptop personal computer, a cellular phone and a car navigation system, it is required that a polarizing plate which constitutes a liquid crystal display device is thin and lightweight. However, in a polarizing plate in which a TAC film and the like are laminated as the protective film as described above, it is difficult to maintain a thickness of the protective film at 20 μm or less from the viewpoint of handling property or durability performance during the operation, so that there is a limitation for the liquid crystal device to be thin and lightweight.
In order to solve the aforementioned problems, a technology has been suggested, in which a transparent thin film layer is formed by coating an active energy ray curable composition on at least one surface of a polarizer. In this case, the active energy ray curable composition which has been suggested may be classified into a radical curable composition and a cationic curable composition according to the curing method.
Meanwhile, when a transparent thin film layer is formed using a cationic curable composition, the transparent thin film layer is advantageous in that excellent adhesion with the polarizer is achieved and water resistance is excellent, but has a structural problem in that it is difficult to secure a uniform degree of curing according to the thickness direction by limitations of a cationic curing method in which the curing rate is slow and the degree of curing is poor, and accordingly, a desired thickness of the transparent protective layer may not be secured.
In contrast, when a transparent thin film layer is formed using a radical curable composition, the transparent thin film layer is advantageous in that there is no problem about the curing non-uniformity in a thickness up to about 100 μm because the curing rate is fast and the degree of curing is excellent, but has a problem in that a hydrophilic functional group generally included for adhesion of a radical curable composition with a PVA device is exposed to the outermost surface of the transparent protective layer and becomes very vulnerable to water resistance, and accordingly, discoloration of the polarizer and the like may occur.
In order to solve the problems, a polarizing plate having a two layer-type transparent thin film layer has been suggested by forming a transparent thin film layer on a polarizer using a cationic curable composition, and forming a transparent thin film layer again using a radical curable composition on the transparent thin film layer formed. Meanwhile, in this case, since the first layer is a cured product of the cationic curable composition, and thus blocks the polarizer through a covalent bond, it is possible to prevent the polarizer from being discolored in a moisture-resistant environment, but the second layer, which is a cured product of a radical curable composition generally including a hydrophilic functional group, is externally exposed to the outermost layer, and thus becomes exposed to moisture, thereby being easily peeled off with the first layer.
Therefore, there is a need for a new polarizing plate, which may form a protective layer in a desired thickness and simultaneously secure excellent water resistance, may significantly reduce the curling generation rate, and may be manufactured as a thin type as compared to a polarizing plate having a transparent protective film in the related art.